1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to circuit boards and differential signal propagation for such circuit boards.
2. Description of the Related Art
Modern processors communicate with peripherals and other devices at sometimes very high data rates and operating frequencies. To accommodate such high data rates, conventional processors often use high density interconnects. Differential signaling is used for many such high density interconnects. In some conventional designs, many differential signal pairs (pairs of signal traces) are mapped in a small space and are forced to use minimum geometries, such as width, thickness and spacing, that can be afforded by prevailing design rules. The minimum geometries are beneficial from the aspect of reduced capacitive loading on transmission lines for high data rates. However, the minimum spacing can lead to high crosstalk issues between adjacent differential pairs.
Some conventional designs address the problem of differential signal pair crosstalk by various filtering techniques, such as pre-shoot, de-emphasis equalization and continuous time linear equalization. These techniques use on-die circuitry to perform the signal conditioning techniques. Of course, there is a performance overhead associated with these techniques and silicon space is consumed.
Differential signal vias, which often traverse one or more layers of a circuit board present certain challenges. A server platform for modern microprocessors have many high speed interfaces, including USB, DisplayPort, PCIe, SATA and DDR. To accommodate these various interfaces, systems boards have grown to ten layers or more for signals and power delivery. Using thin materials, such as thin metal layer and low k dielectrics, can somehow reduce the overall platform thickness while keeping the impedance constant. The cost of using thin and low k plastic materials is high and thin metal layers can lead to DC IR drop penalty. Another issue for relatively thick boards is that long via transitions cause high far end crosstalk. The far end crosstalk induced by electrical and magnetic couplings is strongly affected by the length of coupling: the thicker the board, the longer the differential signal transition vias. The crosstalk also gets worse when high speed differential signals are crowded together, particularly when they share the same return current. When the return path is shared by several differential pairs, the return currents for them overlap one on top of each other. Indeed, the return current overlap is a fundamental factor of crosstalk. Traditionally, the one ground via is shared by two or more nearby differential pairs. Each pair draws its own return current that overlaps on the same ground via surface.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.